First observation of the decay Λb0 →ηc (1S)pK-

Abstract

The decay Λb0→ηc(1S)pK- is observed for the first time using a data sample of proton-proton collisions, corresponding to an integrated luminosity of 5.5 fb-1, collected with the LHCb experiment at a center-of-mass energy of 13 TeV. The branching fraction of the decay is measured, using the Λb0→J/ψpK- decay as a normalization mode, to be B(Λb0→ηc(1S)pK-)=(1.06±0.16±0.06-0.19+0.22)×10-4, where the quoted uncertainties are statistical, systematic and due to external inputs, respectively. A study of the ηc(1S)p mass spectrum is performed to search for the Pc(4312)+ pentaquark state. No evidence is observed and an upper limit of B(Λb0→Pc(4312)+K-)×B(Pc(4312)+→ηc(1S)p)B(Λb0→ηc(1S)pK-)<0.24 is obtained at the 95% confidence level.